Isomerization of 2-cis-pentenoates to 2-trans-pentenoates

According to German Laid-Open Application DOS No. 3,317,163, 2-cis- and 2-trans-pentenoates are obtained as byproducts in the isomerization of 3-pentenoates to 4-pentenoates in the presence of catalysts containing palladium, rhodium or ruthenium. Methyl 2-cis-pentenoate has the same boiling point as methyl 4-pentenoate and therefore cannot be separated from the latter by distillation.

It is an object of the present invention to convert 2-cis-pentenoates to isomers whose boiling point allows them to be separated from 4-pentenoates by distillation, any 4-pentenoate present not being converted.

We have found that this object is achieved by a process for the isomerization of 2-cis-pentenoates to 2-trans-pentenoates, wherein a 2-cis-pentenoate is treated at from 20° to 250° C. in the presence of one or more compounds of the formula I ##STR3## where R 1 is hydrogen or R 1 and R 2 are each alkyl of 1 to 6 carbon atoms or cycloalkyl of 5 to 8 carbon atoms, and R 1 and R 2 together with the nitrogen atom on which they are substituents may form a 5-membered to 7-membered ring which may additionally contain a further nitrogen or oxygen atom, and R 3 is hydrogen or a radical of the formula II ##STR4## where R 4 is alkyl of 1 to 6 carbon atoms, cycloalkyl of 5 to 7 carbon atoms, aralkyl of 7 to 10 carbon atoms or aryl of 6 to 10 carbon atoms, with the proviso that a tertiary amine is present only when R 3 is a radical of the formula II.

The novel process has the advantages that 2-cis-pentenoates can be converted in a simple manner to 2-trans-pentenoates, which, because of their boiling point, can readily be separated from the corresponding 4-pentenoates by distillation, and that 4-pentenoates present during the isomerization undergo virtually no conversion.

Preferred 2-cis-pentenoates are derived from alkanols of 1 to 6 carbon atoms, cycloalkanols of 5 to 7 carbon atoms, aralkanols of 7 to 10 carbon atoms or phenol or naphthol. The corresponding alkyl esters have become particularly important, in particular those of alkanols of not more than 3 carbon atoms. Examples of suitable starting compounds are methyl, ethyl, n-propyl, isopropyl, tert-butyl, isobutyl, n-butyl, sec-butyl, n-pentyl, n-hexyl, dodecyl, cyclopentyl, cyclohexyl, cycloheptyl, benzyl, phenylethyl and phenyl 2-cis-pentenoate. The 2-cis-pentenoates may also be present as a mixture with the corresponding 4-, 3-cis- and 3-trans- and/or 2-trans-pentenoates. Typical mixtures contain, for example, from 0.1 to 99.5% by weight of 4-pentenoate, from 0.1 to 99.5% by weight of 3-cis- and trans-pentenoate, from 0.1 to 99.5% by weight of 2-trans-pentenoate and from 0.1 to 100% by weight of 2-cis-pentenoate.

The isomerization is carried out in the presence of one or more compounds of the formula I, where R 1 is hydrogen or R 1 and R 2 are each cycloalkyl of 5 to 8 carbon atoms or alkyl of 1 to 6 carbon atoms, and R 1 and R 2 together with the nitrogen atom on which they are substituents may form a 5-membered to 7-membered ring which additionally may contain a further nitrogen or oxygen atom, and R 3 is hydrogen or a radical of the formula II, where R 4 is alkyl of 1 to 6 carbon atoms, cycloalkyl of 5 to 7 carbon atoms, aralkyl of 7 to 10 carbon atoms or aryl of 6 to 10 carbon atoms, with the proviso that a tertiary amine is present only when R3 is a radical of the formula II.

Examples of suitable compounds are primary or secondary amines of the formula I, such as methylamine, ethylamine, n-butylamine, isobutylamine, n-hexylamine, cyclohexylamine, cyclopentylamine, di-n-butylamine or dicyclohexylamine.

In preferred amines of the formula I, R 3 is hydrogen and R 1 and R 2 together with the nitrogen atom on which they are substituents form a 5-membered to 7-membered ring which may additionally contain a nitrogen or oxygen atom. Examples of these are pyrrolidine, piperidine, piperazine, morpholine and hexamethyleneimine.

Other preferred catalysts are tertiary amines of the formula I, where R 1 and R 2 form a 5-membered or 6-membered ring which may furthermore contain an oxygen or nitrogen atom and R 3 is a radical of the formula II, when R 4 is, in particular, alkyl of 1 to 6 carbon atoms. Examples of suitable compounds are methyl 3-piperidino-, 3-pyrrolidino-, 3-morpholino- and 3-piperazinovalerate, isopropyl 3-piperidinovalerate, cyclohexyl 3-pyrrolidinovalerate, benzyl 3-piperazinovalerate and phenyl 3-morpholinovalerate.

From 0.01 to 1, in particular from 0.05 to 0.2, mole of the compound of the formula I is advantageously used per mole of 2-cis-pentenoate.

The isomerization is carried out at from 20° to 250° C., in particular from 50° to 200° C., as a rule under atmospheric pressure. However, it is also possible to employ reduced pressure or slightly superatmospheric pressure, for example up to 10 bar. The isomerization is generally effected batchwise or continuously, for example in a homogeneous liquid phase.

Although the isomerization is carried out in general in the absence of solvents, it is also possible to use organic solvents which are inert under the reaction conditions, such as aliphatic or cycloaliphatic hydrocarbons, eg. pentanes, hexanes, cyclohexane, aromatic hydrocarbons, eg. benzene, toluene or xylenes, and ethers, eg. methyl tert-butyl ether, dioxane or tetrahydrofuran, or esters, such as ethyl acetate.

The batchwise isomerization of 2-cis- to 2-transpentenoates is carried out, for example, as follows: the 2-cis-pentenoate, if desired as a mixture with 2-trans-, 3- and/or 4-pentenoates, is heated to the stated temperature together with the abovementioned amount of a compound of the formula I. After a reaction time of, for example, from 0.5 to 5 hours, the catalyst I is separated from the pentenoate isomer mixture, advantageously by distillation. The compound I used as a catalyst can be recycled to the isomerization stage. The pentenoate mixture is, as a rule, then worked up by distillation.

The novel process permits, for example, methyl 2-cis-pentenoate to be converted to methyl 2-trans-pentenoate and thus into a form which can be separated from methyl 4-pentenoate by distillation.

The 2-trans-pentenoates obtainable by the novel process can be isomerized to 3-pentenoates, which are important starting materials for the preparation of adipic acid.

Isomerization of methyl 2-cis-pentenoate with piperidine in the presence of methyl 2-trans-, 3- and 4-pentenoate

A mixture of 2.3 g of methyl 2-cis-pentenoate, 2.3 g of methyl 2-trans-pentenoate, 16 g of methyl 3-pentenoate and 2.3 g of methyl 4-pentenoate (GC: 10.0% by area of methyl 2-cis-pentenoate, 10.4% by area of methyl 2-trans-pentenoate, 67.9% by area of methyl 3-pentenoate and 10.2% by area of methyl 4-pentenoate) was heated with 1.7 g of piperidine for 4 hours at 65° C. Analysis of the pentenoates by gas chromatography (% by area) after this time indicated that the reaction mixture contained 1.0% of methyl 2-cis-pentenoate, 19.5% of methyl 2-trans-pentenoate, 66.5% of methyl 3-pentenoate and 10.1% of methyl 4-pentenoate.

Isomerization of methyl 2-cis-pentenoate with piperidine in the presence of methyl 4-pentenoate

A mixture of 2.3 g of methyl 2-cis-pentenoate and 20.5 g of methyl 4-pentenoate was heated together with 1.7 g of piperidine for 4 hours at 65° C. Analysis (% by area) of the pentenoates by gas chromatography after this time indicated that the reaction mixture contained 0.8% of methyl 2-cis-pentenoate, 9.0% of methyl 2-trans-pentenoate, 0.1% of methyl 3-cis-+trans-pentenoate and 89.8% of methyl 4-pentenoate.

EXAMPLES 3 TO 8

Examples 3 to 8 were carried out as described in Examples 1 and 2. Table 1 contains the starting materials and amounts, reaction conditions and gas chromatographic analyses. 20 g of pentenoate isomer mixture were used in each example.

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PAE Catalyst

Reaction

Reaction

mixture.sup.(1)

[mol % based

temperature

time GC [% by area]

No. used on 2-cis-PAE]

[°C.]

[h] 2-cis-PAE

2-trans-PAE

3-PAE

4-PAE

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3 20% 2-cis-,

n-Hexylamine

130 4 8.5 11.2 0.3 78.5

80% 4- 10

4 Cyclohexyl-

190 3 4.4 14.0 1.9 79.3

amine

10

5 30% 2-cis-,

Piperazine

180 4 7.6 21.5 1.3 69.7

70% 4- 1

6 20% 2-cis-,

Morpholine

160 4 7.7 12.2 0.3 79.3

80% 4- 5

7 Pyrrolidine

160 2 2.6 16.7 0.8 79.2

5

8 30% 2-cis-,

Di-n-butyl-

140 4 20.7 6.9 2.5 69.8

70% 4- amine

10

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.sup.(1) PAE = methyl pentenoate